[ALSA] mixart: Use SEEK_{SET,CUR,END} instead of hardcoded values

mixart: Use SEEK_{SET,CUR,END} instead of hardcoded values

Signed-off-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>